The invention relates to a ventilating device, comprising a fan, an electromotor coupled with the fan for driving the latter, a controllable excitation circuit coupled with the motor, and means for detecting the rotational speed of the fan coupled with the excitation circuit.
Ventilating devices of this type widely are used in practice, e.g. for air refreshing purposes, whereby exhaust or polluted air or other gases are removed from a room and/or whereby clean air is supplied to a room. Other applications consist in removing hot air, supplying gas or air to combustion ovens and the like.
In some industrial applications, especially in the agricultural sector, the ventilating device and the direct vicinity thereof can be polluted by dust deposit and other pollution to such an extent, that the fan finally gets blocked. It is also possible, that such big parts are sucked out of the environment, that this causes a blocked fan as well. Standstill of the motor due to a blocked rotor generally leads to relatively high currents in the motor winding and the supply lines of the electrical installation. This can cause damage to the excitation circuit of the motor and an undesired heat-development in the motor and in the electrical installation, which leads to an increased fire risk.
In practice such an erroneous situation is usually detected by means of overload means and/or short-circuit means, whereby the excitation circuit is switched off. The U.S. Pat. No. 4,513,233 discloses an electronic monitoring circuit, suitable for this purpose, provided for monitoring the rotational speed of the electromotor.
Since, in practice, it is further common use to control several ventilating devices with one separate excitation circuit, as a consequence the occurrence of an erroneous situation in one ventilating device can disturb the functioning of the other ventilating devices as well. To prevent this, in practice, the separate ventilating devices are provided with a motor protection switch, which protects the motor of a ventilating device against overload currents and/or short-circuit. However, this leads to a more complicated and hence more expensive electrical installation.